This invention relates to a process for the preparation of amino-1,3-benzenediols and to novel intermediates used in their preparation.
Diaminobenzenediols and monoaminobenzenediols are useful as monomers for the preparation of polybenzoxazoles. Polybenzoxazoles can be prepared by reacting diaminodihydroxybenzenes with bisacids, bisacid halides, bisesters or bisnitriles. In order to obtain a high molecular weight polybenzoxazole which can be effectively spun into workable fibers, it is necessary that the starting materials used to form the polybenzoxazoles are of very high purity. Polybenzoxazoles prepared from highly pure diaminobenzenediols can be spun into fibers having high tensile strength and thermal stability. Such fibers are desirable for military, aerospace and other applications requiring high performance materials.
The traditional method for preparing 1,3-diamino-4,6-dihydroxybenzene involves the treatment of 1,3-diacetoxybenzene with white nitric acid. The treatment with nitric acid results in the formation of the undesirable 2,4,6-trinitro-1,3-benzenediol and 2,4-dinitro-1,3-benzenediol. Repeated recrystallizations are required to isolate the desired 4,6-dinitro-1,3-benzenediol from the undesirable by-products. The 4,6-dinitro-1,3-benzenediol is catalytically hydrogenated in dilute hydrochloric acid to produce the 4,6-diamino-1,3-benzenediol. See Wolfe et al., Macromolecules, 14, p. 909 (1981). This process is disadvantageous in that it requires extensive purification and utilizes expensive starting materials.
Monoamino benzenediols are known to be useful as materials for making dyes and are made by procedures similar to making diaminobenzenediols and which similarly suffer from the same deficiencies for making diaminobenzenediols.
What is needed is an economical high yield process which results in the formation of a substantially pure amino-1,3-benzenediol. Such a process would provide for the efficient production of amino-1,3-benzenediols which could be used to form the desirable high molecular weight polybenzoxazoles.